Solar cell degradation can occur through many pathways and at the different stages of the fabrication process, notably because of the water condensation. In the case of Cu(In,Ga)Se ₂ (CIGS) solar cells, the impact of water ingress on Mo back contact can be substantial for not only the film itself but also the device performance and reliability. Microstructural modification, change in film morphology, loss in reflectance, and increased resistivity because of the moisture ingress were observed via X-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry, and four-point probe measurements, respectively. Secondary ion mass spectrometry measurements revealed drastic changes in the alkali (Na, K) profiles in both the CIGS and Mo layers, likely because of the modification of their diffusion coefficient through Mo. This, in turn, negatively impacts the solar cell efficiency by decreasing both fill factor and open-circuit voltage. Additional experiments, modifying the substrate and utilizing a NaF postdeposition treatment, highlight the mechanisms of degradation as being due to both a modification of the Mo/CIGS interface and the lack of alkali diffusion after water ingress.

The impact of moisture and heat treatment on the microstructural, chemical, and electrical properties of Cu(In, Ga)Se $_2$ films and their collective effect on the solar cell device performance was studied. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements show that water exposure causes surface modification and alters the alkali metal distribution, while no composition or structural effect was observed. Deep level transient and optical spectroscopies revealed that the trap densities (N $_T$ ) for both the E $_V$ + 0.65 eV and E $_V$ + 0.98 eV traps increase after water exposure, while the majority carrier concentration (N $_A$ ) decreases. Time-resolved photoluminescence (PL) and steady-state PL measurements indicated the presence of static, not dynamic, quenching. Reduction of open-circuit voltage (V $_OC$ ) and fill factor (FF) was observed for the devices but was not associated with a change of recombination mechanism, which remains in the absorber space charge region. A small increase in series resistance and shunt conductance accounts for most of the FF change, while the modification in both N $_A$ and N $_T$ yield most of the change in V $_OC$ . A gradient of majority carrier concentration, related to the alkali profile, also yields a small voltage-dependent current collection after moisture and heat treatment.